Porcelain is one of the most important materials used in dentistry. It lends itself to the manufacture of the most esthetic dental restorations since it can be colored to closely resemble the teeth it must replace.
Porcelain exhibits excellent chemical qualities insofar as dental applications are concerned. It is insoluble in the normal fluids of the oral cavity and in practically any given food or drink likely to pass through the oral cavity. It is also chemically able to resist the acid or alkali materials frequently used for washing artifical teeth. Moreover, mammalian tissues are very tolerant of its presence and such tolerance remains even after years of continuous contact.
Porcelain does have, however, one great disadvantage. It is relatively fragile and repairs are difficult and costly. Because of the hazard of fragility, artificial dental crowns and bridges have heretofore been made using a metallic framework coated with a fused dental porcelain to provide the desired esthetics and strength.
The type of porcelain that is currently most often employed in dental restorations is typified by that described in the Weinstein et al patents, U.S. Pat. Nos. 3,052,982 and 3,052,983. The Weinstein et al patents address the problem of preparing a porcelain whose coefficient of thermal expansion will match that of the metal base so that excessive stress formation will not occur during the production of the restoration.
The solution proposed by Weinstein et al was to make a dental porcelain composed of two different frits, one having a high coefficient of expansion and the other having a much lower coefficient of expansion, to result in a porcelain having a coefficient of expansion intermediate between the two materials, and which will match the dental alloy employed as the base.
The major disadvantage of the metal supported porcelain restoration is the loss of translucency which is especially noticeable at the gingival margin area. The junction of the restoration with the tooth may lack the translucent esthetic quality desired even in the case of an all-porcelain margin. Also, the added complexity of waxing, casting and metal finishing requires an increased amount of labor for the production of the restoration. Still, it has been the most versatile restoration heretofore employed.
Mechanisms of strengthening ceramics, other than the use of metal ceramic systems, have primarily involved dispersion strengthening (aluminous core materials) and controlled crystallization (Dicor.RTM. castable glass ceramic from Dentsply International, Inc., York, Pa., and CeraPearl.RTM. castable glass ceramic from Kyocera, Inc., San Diego, Calif.). The aforementioned systems are not greatly different in their clinical strength. Color is applied to the surface of such cast glasses, which limits their potential for optimum esthetics.
An alternative to the cast-glass ceramic systems is the extrusion molded system which employs an epoxy die and a shrink-free (expansion/contraction controlled) system, for example, Cerestore.RTM. nonshrink alumina ceramic of Johnson & Johnson Dental Products, Inc. The extrusion molded and cast-glass ceramic systems exhibit disadvantages, including a high initial equipment cost and the fact that each system is somewhat labor intensive.
A further disadvantage of systems employing an aluminous core is the reduced translucency produced by the semi-opaque nature of the core materials. Both the cast ceramic systems and the dispersion strengthened systems also have an inherent disadvantage in the manner in which they might be joined to form multiple units. Satisfactory commercially feasible systems of joining alumina reinforced ceramic units have not been developed.
Accordingly, it is an object of the present invention to provide a high strength dental porcelain for use in making all-ceramic dentures, crowns and bridges, thereby obviating the need for a metal or ceramic support.
A further object of the present invention is to provide a dental porcelain composition which is translucent and exhibits the ability to accept colors producing a restoration exhibiting desirable dental shades.
A still further object of this invention is to provide artifical crowns and bridges with greater impact strength and hence greater resistance to chipping.
A still further object is to provide a high strength dental porcelain composition which can be used employing present laboratory equipment and eliminating the need for extensive heat treatment.
A still further object is to provide permanent dental restorations exhibiting high strength, i.e., a minimum compressive strength of at least about 125,000 p.s.i., a diametral tensile strength of at least about 6,000 p.s.i. and a flexural strength of at least about 16,000 p.s.i.